In general, continuous ink jet printing apparatus have a print head manifold cavity to which ink is supplied under pressure so as to issue in a streams from a print head orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid streams, e.g. vibrations by an electromechanical transducer, to cause the streams to break-up into uniformly sized and shaped droplets. A charge plate, comprising an array of addressable electrodes, is located proximate the streams break-off points to induce an electrical charge, selectively, on adjacent droplets, in accord with print information signals. Charged droplets are deflected from their nominal trajectory; e.g. in one common (binary) printing mode, charged (non-print) droplets are deflected into a catcher device and non-charged droplets proceed to the print medium.
A number of different catcher devices have been developed as constructions to intercept and recirculate the non-print droplets from such print heads. The catcher devices must take several potential problems into account. First, the catcher device must intercept the non-print ink droplets in a way that avoids splattering them onto the print medium, or scattering into an ink mist, which also can cause defects on the print media. Second, the catcher devices must effectively remove the caught ink away from the droplet interception zone so that a build-up of ink on the catching surface does not block the flight path of printing drops.
To accomplish these purposes, one prior art approach provides catcher devices with a drop impact surface generally parallel to the print drop trajectory and provides a drop discharge channel below the drop impact surface. Typically, a vacuum source is coupled to the drop discharge channel to urge a uniform ink discharge flow, from the impact surface to a channel egress. To enhance the uniformity of ink discharge flow, grooves and ridges, extending in the direction of desired flows, have been provided on the catcher impact surface and in the drop discharge channel (see U.S. Pat. Nos. 3,813,675 and 3,836,914).
U.S. Pat. No. 4,035,811 is exemplary of another prior art catcher feature in its provision of a porous drop discharge channel wall, which ingests stray ink droplets.
The above and other catcher constructions perform adequately where the catcher is not moving during the print operation and where the droplet stream is vertical (so that ink in the discharge channel is not subjected to transverse gravitational forces). However, when the catcher is part of a print head assembly acceleration forces can cause ink at its discharge channel ingress to be slung away from the catcher. Slung ink masses can appear on the print media as defects or contaminate the machine. Even where the acceleration forces are not sufficient to sling the ink, they can cause dynamic buckling of the ink film just entering the discharge channel ingress. The buckled ink film can obstruct ink droplets which should pass to the print media, which will cause splatter and/or "white defects", as a result of the droplet interception.
Also, in applications where it is desirable to dispose the print head at various orientations (e.g. along a bindery line), the prior art catcher devices do not perform properly. That is, when a print head is disposed with the line direction of its orifice array non-parallel to the horizontal (e.g. tilted at 45.degree. or 90.degree.), the catcher ingress throat and the ink discharge flow path are correspondingly tilted. In such orientations it has been observed that gravity causes ink build-up along the "low" sides of the catcher throat and discharge channel. This eventually causes ink to drip from the catcher ingress throat.